The quality of data reported by a strain gauge mounted to a metallic substrate depends on the integrity of the adhesive bond between the strain sensor and the substrate. It is generally accepted that the adhesive bond (typically an epoxy) breaks down in the presence of moisture. Swelling of the epoxy due to moisture absorption results in shear stresses at the epoxy/metal interface, and over time, these shear stresses can result in failure of the epoxy bond and de-lamination of the strain gauge.
One solution to this problem, often employed on large civil structures, is to package the strain gauge within a sandwich of two hermetically sealed stainless steel ribbons. Laser or electron beam is used to provide the sealing. This strain sensitive ribbon is then spot welded to the structure under test. However, this spot welding process creates localized changes in the steel's microstructure which may be subject to higher than normal rates of corrosion. For many applications of welded structures, the creation of corrosion focus points is considered unacceptable, as these could result in degradation in the physical appearance, added maintenance costs, or even the initiation of material failure. Therefore protection against moisture is desired.
None of the systems for connecting a strain sensor to a structure have been satisfactory in providing a reliable bond that is resistant to moisture degradation without affecting structural properties. In addition, when moisture degradation occurs there has been no way to recognize that data coming from the sensor is not acceptable. Thus, a better system for connecting strain sensors to structures is needed, and this solution is provided by the following.